Measurement of disease frequency is basic requirement of epidemiological study. Measures of disease frequency are ratio, proportion and rate. Most frequently used measures are Incidence and prevalence. Incidence can be measured from cohort studies. It is of two types-cumulative incidence and incidence density. Prevalence depends on incidence and duration of disease. There are two kinds—point prevalence and period prevalence. A measure of association in case control studies is Odds ratio. In cohort studies association is measured by Relative risk and Attributable risk.